Local Friction Coefficient Research Articles

Effect of displacement loading rate on dynamic mechanical characteristics of fault stick-slip under constant normal stiffness conditions

The role of the displacement loading rate in fault activation and evolution is of utmost importance, yet most existing studies have primarily focused on its effect under constant normal load (CNL) conditions. However, in reality, the rock mass surrounding the earthquake source at the deep part of the fault provides normal stiffness conditions. Therefore, the influence of the displacement loading rate on the dynamic mechanical characteristics of fault stick-slip under constant normal stiffness (CNS) conditions remains uncertain. To bridge this research gap, we conducted an experimental investigation on simulated faults in granite under constant normal stiffness (CNS) conditions. Our analysis focused on the influence of the displacement loading rate on the spatial and temporal evolution of local stresses along the fault and the size of the fault nucleation zone. The findings revealed the following: Under the CNS condition, the equivalent shear stress is larger compared to the CNL condition, while the shear stress drop is smaller. The recovery of normal stress under CNS conditions depends more on the displacement loading rate. The nucleation location of the fault is influenced by the normal boundary conditions, with a larger nucleation zone observed in the CNS condition than in the CNL condition. In the early stage of friction, the local stress and stress drop of the fault exhibit a positive correlation with the displacement loading rate, whereas in the later stage, they display a negative correlation. Moreover, at the same location, the order of local stress drop rates for the fault is v3 > v1 > v2, where v1 represents the local normal stress drop rate, v2 represents the local shear stress drop rate and v3 represents the local friction coefficient drop rate. These research results contribute to a further understanding of the deep fault-slip mechanical behavior.

Hybrid ferrofluid flow on a stretching sheet with Stefan blowing and magnetic polarization effects in a porous medium

PurposeThis study investigates the behavior of viscous hybrid ferromagnetic fluids flowing through plain elastic sheets with the magnetic polarization effect. It examines flow in a porous medium using Stefan blowing and utilizes a versatile hybrid ferrofluid containing MnZnFe2O4 and Fe3O4 nanoparticles in the C2H2F4 base fluid, offering potential real-world applications. The study focuses on steady, laminar and viscous incompressible flow, analyzing heat and mass transfer aspects, including thermal radiation, Brownian motion, thermophoresis and viscous dissipation with convective boundary condition.Design/methodology/approachThe governing expression of the flow model is addressed with pertinent non-dimensional transformations, and the finite element method solves the obtained system of ordinary differential equations.FindingsThe variations in fluid velocity, temperature and concentration profiles against all the physical parameters are analyzed through their graphical view. The association of these parameters with local surface friction coefficient, Nusselt number and Sherwood number is examined with the numerical data in a table.Originality/valueThis work extends previous research on ferrofluid flow, investigating unexplored parameters and offering valuable insights with potential engineering, industrial and medical implications. It introduces a novel approach that uses mathematical simplification techniques and the finite element method for the solution.

Numerical Computational of Chemically Reactive Hybrid Nanofluids with Melting Phenomena Passing Through a Disk: Comparative Study

This paper aims to investigate the solutions for the axisymmetric flow and heat transfer coming from a permeable disk in hybrid nanofluids. The nanofluids are under the influence of thermal radiation and contain magnetohydrodynamics and melting phenomena. For this, the momentum and temperature mathematical model is developed to investigate the axisymmetric flow of two-dimensional hybrid nanofluids, containing nanoparticles and a base fluid. Using appropriate similarity variables, nonlinear partial differential equations have been transformed into ordinary differential equations. These are further solved using the function bvp4c, which is built into MATLAB software. The physical behavior of parameters is discussed for the values on the basis of visuals and tables. The analysis further shows an increase in the local Nusselt number and skin frictional coefficient due to nonlinear thermal radiation and magnetic parameters. The results may be promising for the applications of hybrid nanofluids in heat transfer and cooling systems of all modern industries. The authors have confidence in their study due to the novelty of the results and underline the numerous practical utilities of hybrid nanofluids.

Effects of Proline on Internal Friction in Simulated Folding Dynamics of Several Alanine-Based α-Helical Peptides.

We have studied in silico the effect of proline, a model cosolvent, on local and global friction coefficients in (un)folding of several typical alanine-based α-helical peptides. Local friction is related to dwell times of a single, ensemble-averaged hydrogen bond (HB) within each peptide. Global friction is related to energy dissipated in a series of configurational changes of each peptide experienced by increasing the number of HBs during folding. Both of these approaches are important in relation to future atomic force microscopic-based measurements of internal friction via force-clamp single-molecule force spectroscopy. Molecular dynamics (MD) simulations for six peptides, namely, ALA5, ALA8, ALA15, ALA21, (AAQAA)3, and H2N-GN(AAQAA)2G-COONH2, have been conducted at 2 and 5 M proline solutions in water. Using previously obtained MD data for these peptides in pure water as well as upgraded theoretical models, we obtained variations of local and global internal friction coefficients as a function of solution viscosity. The results showed the substantial role of proline in stabilizing the folded state and slowing the overall folding dynamics. Consequently, larger friction coefficients were obtained at larger viscosities. The local and global internal friction, i.e., respective, friction coefficients approximated to zero viscosity, was also obtained. The evolution of friction coefficients with viscosity was weakly dependent on the number of concurrent folding pathways but was rather dominated by a stabilizing effect of proline on the folded states. Obtained values of local and global internal friction showed qualitatively similar results and a clear dependency on the structure of the studied peptide.

Stochastic thermodynamics of Brownian motion in temperature gradient

We study stochastic thermodynamics of a Brownian particle which is subjected to a temperature gradient and is confined by an external potential. We first formulate an over-damped Ito-Langevin theory in terms of local temperature, friction coefficient, and steady state distribution, all of which are experimentally measurable. We then study the associated stochastic thermodynamics theory. We analyze the excess entropy production both at trajectory level and at ensemble level, and derive the Clausius inequality as well as the transient fluctuation theorem (FT). We also use molecular dynamics to simulate a Brownian particle inside a Lennard-Jones fluid and verify the FT. Remarkably we find that the FT remains valid even in the under-damped regime. We explain the possible mechanism underlying this surprising result.

Forced convection analysis of Williamson-based magnetized hybrid nanofluid flow through a porous medium: Nonsimilar modeling

The purpose of this work is to analyze the convective horizontal flow of Williamson-based magnetized hybrid nanofluid. The flow of Williamson fluid with heat transfer is stimulated in a porous media over a stretching surface. Two different type of nanoparticles such as, molybdenum disulfide ( Mo S 2 ) and copper (Cu) with engine oil (Eo) as base fluid are considered in this work. The convective flow of Williamson fluid is inspected based on Khanefer model. The governing system is transformed into nondimensionalized partial differential equations (PDE’s) by utilizing nonsimilar transformation. The transformed system is analytically approximated using local nonsimilarity (LNS) and perturbation approach which approximate the nondimensional PDE’s by nondimensional ordinary differential equations (ODEs). The LNS and perturbation ODEs are simulated utilizing finite difference-based algorithm bvp4c. The outcomes of simulation for heat transfer and convective flow over stretching surface incorporating the evolvement with local Nusselt number (Nu) for distinct values of prandtl number are compared with previous published numerical results. The appropriate range of governing nondimensionalized parameters is evaluated to detect the variance of physical quantities namely, velocity ( f ′ ( η ) ) of the fluid, temperature ( θ ′ ( η ) ) of the fluid, co-efficient of local Nusselt number (Nu) and co-efficient of local skin friction (Cf). The percentage difference between single nano-fluid and hybrid nano-fluids is made which are given in tabular form. To the best of author knowledge, nonosimilar study of hybrid-based magnetized Williamson nanofluid is not yet investigated. This work is anticipated to offer crucial information for the implementation of innovative heat transfer devices in the future and serve as an invaluable resource for researchers looking at the flow of nanofluids under various assumptions.

Verification of the γ-Reθt Transition Model in OVERFLOW and FUN3D

The findings of the recent AIAA and NATO workshops on transition modeling have underscored the importance of verifying the transition models that are based on Reynolds-averaged Navier–Stokes. To that end, the present work aims to verify the Langtry–Menter (LM2009) transition model coupled with Menter’s shear-stress transport (SST) turbulence model. Specifically, the SST-2003-LM2009 model, as implemented in the OVERFLOW structured overset solver and the FUN3D unstructured solver, is applied to both natural and bypass transition scenarios on a flat plate and to natural and separation-bubble-induced transition on the NLF-0416 airfoil. The Richardson extrapolation method was used to analyze and report the grid convergence of integrated and local load coefficients upstream, within, and downstream of the transition region, based on solutions obtained using the two solvers on identical grids. Reasonable agreement is obtained between the two codes for both global and local surface load coefficients when further refining the workshop mesh series, although with a grid-convergence index for the local friction coefficient that may exceed 1% in certain cases. The study also highlights the solution sensitivity to the choice of turbulence inflow boundary conditions and model options, along with the necessity to unambiguously specify this information for benchmark exercises.

Effect of electrostatic force and thermal radiation of viscoelastic nanofluid flow with motile microorganisms surrounded by PST and PHF: Bacillus anthracis in biological applications

Current studies have a significant application in various fields; motile microorganism, such as bacteria, can be used for targeted drug delivery. This is because they can be engineered to carry drugs to specific locations in the body. For example, bacteria can be modified to express genes that code for targeting ligands, which are molecules that bind to specific receptors on cells. This could help to prevent the bacteria from multiplying and causing disease. The main aim of the current work is to perform the numerical analysis of three-dimensional radiative, steady viscoelastic nanofluids flow towards an exponentially stretchable porous surface. The impact of nth-order chemical reaction and motile microbes are also disclosed looks at how multiple slips affect the Buongiorno model for magnetohydrodynamic viscoelastic nanofluids with radiation across a permeable stretched sheet. The appropriate suitable are used to transform nonlinear partial differential equations into ordinary differential equations. The numerical solution of the resulting system of equations is handled numerically by employing the bvp4c algorithm built-in MATLAB Software which comes of three-stage Lobatto IIIa formula. Dimensionless values such as temperature, concentration, velocity, and the non-Newtonian nano-fluid density profile are explored, as are non-dimensional numbers such as the local Nusselt, local friction coefficient, Sherwood, and motile microbe. Present results signifies enhancement in the temperature profiles as well as the thickness of the thermal boundary layer by increasing Brownian motion (Nb) and thermophoresis parameters (Nt), whereas decrement is noted against the viscoelastic parameter (K). As the value of the magnetic parameter ranges 0.0≤M≤ 0.2 the relative increment noted in skin friction coefficient is about 1.9 %, while decrement is found in curves of velocity field. Our findings are compared with the data available in the literature and found to be in good consensus.

Impact of Indenter Size and Microrelief Anisotropy on the Tribological Behavior of Human Skin.

Everyday, we interact with screens, sensors, and many other devices through contact with the skin. Experimental efforts have increased our knowledge of skin tribology but are challenged by the fact that skin has a complex structure, undergoes finite deformations, has nonlinear material response, and has properties that vary with anatomical location, age, sex, and environmental conditions. Computational models are powerful tools to dissect the individual contribution of these variables to the overall frictional response. Here, we present a three-dimensional high-fidelity multilayer skin computational model including a detailed surface topography or skin microrelief. Four variables are explored: local coefficient of friction (COF), indenter size, mechanical properties of the stratum corneum, and displacement direction. The results indicate that the global COF depends nonlinearly on the local COF, implying a role for skin deformation on the friction response. The global COF is also influenced by the ratio of the indenter size to the microrelief features, with larger indenters smoothing out the role of skin topography. Changes in stiffness of the uppermost layer of skin associated with humidity have a substantial effect on both the contact area and the reaction forces, but the overall changes in the COF are small. Finally, for the microrelief tested, the response can be considered isotropic. We anticipate that this model and results will enable the design of materials and devices for a desired interaction against skin.

Numerical Study of the Effects of Roughness Coupled with Inclination on a Turbulent Flow around an Obstacle

In this study, we simulate the cooling of a microprocessor by thermal convection in three different shapes: a square, a trapezoidal, and a triangular shape. The latter is improved by a variety of types of roughness, including square roughness, triangular roughness Type 1, triangular roughness Type 2, and triangular roughness Type 3. The microprocessors are kept at a constant temperature, the air flow is constant, and the geometry is fixed. The physical phenomenon is simulated by the ANSYS software. The numerical results reported in this study cover the ranges of the obstacle’s angle of inclination, 0°≤θ≤45°, (square obstacles, θ=0°, trapezoidal obstacles, 0°<θ<45°, triangular obstacles, θ=45°) and Reynolds number, 2500≤Re≤10,000. The findings relate to streamlines, dynamic pressure (max), mean velocity, temperature field, mean Nusselt number (Nu/Nu0) profiles, local coefficient of friction (Cf/f0), mean coefficient of friction (f/f0) profiles, mean velocity field with roughness, and fluid temperature field with roughness. The aim of the study is to show the interaction between the roughness parameter and the obstacle geometry. In the case of a triangular obstacle, the contact between the cold air and the obstacle is significant downstream of the obstacle, which gives us good cooling, and the Nusselt number has an important value because the agitation of the flow increases convective heat transfer, and the coefficient of friction is low because the air flow is uniform.

A polytopal method for the Brinkman problem robust in all regimes

In this work we develop a discretisation method for the Brinkman problem that is uniformly well-behaved in all regimes (as identified by a local dimensionless number with the meaning of a friction coefficient) and supports general meshes as well as arbitrary approximation orders. The method is obtained combining ideas from the Hybrid High-Order and Discrete de Rham methods, and its robustness rests on a potential reconstruction and stabilisation terms that change in nature according to the value of the local friction coefficient. We derive error estimates that, thanks to the presence of cut-off factors, are valid across all the regimes and provide extensive numerical validation.

Reibkoeffizientenermittlung in der Zerspanung auf Basis von Hochgeschwindigkeitsaufnahmen

The friction at the cutting wedge has a significant influence on tool wear. Due to the strong local variance of temperatures and stresses at the cutting wedge, the friction conditions differ significantly locally. In this work, a method is presented with which normal stress es, tangential stresses and local friction coefficients at the cutting wedge can be determined based on experimental investigations. For this purpose, high-speed recordings and force measurements are conducted on a planing test rig. In addition to dry cutting processes, investigations are carried out on the test rig using emulsion and oil as metal working fluid. The results show a reduction of the coefficients of friction when oil is used as metal working fluid. However, when emulsion is used as metal working fluid, the coefficient of friction changes only slightly compared to dry machining. This can be attributed to low film-forming ability of the emulsion.

Exploiting surface textures dynamics for dry friction control

We study the dynamic behavior of a lattice of bristle-like elastic elements disposed at the interface between a rigid still substrate and a rigid sliding slab, in steady conditions. Due to normal and frictional interactions with the moving slab, complex bristles dynamics occur, which may eventually alter the overall frictional response of the structured interface. Indeed, up to three main mechanisms of friction control can be identified, depending on the specific bristles dynamics: the relative velocity-dependent modulation of local friction force; the misalignment between the local relative velocity and the slab velocity, due to the emergence of transverse vibration; the local friction coefficient variation due to the normal load acting on the bristle. Results show that, depending on the interface dynamic properties (i.e., bristles stiffness, normal load, slab velocity, etc.), a significant reduction of the friction force opposing the slab motion can be achieved, also involving self-excited bristle vibration. Since the present formulation is scale independent, this result may suggest possible mechanisms of friction control in different practical application fields, ranging from bio-inspired micro-structured interfaces to macro-scale features, such as brush seals in electric motors.

Turbulence and thermo‐flow behavior of air in a rectangular channel with partially inclined baffles

AbstractIn this paper, the thermal performance improvements of a heat removal system like an electronic system have been analyzed. The studied case is a horizontal channel in which two partially inclined baffles are attached with variable height and number. The channel is crossed by a forced convective flow of a cooling fluid (air). This numerical work evaluates the influences of the height and number of the baffles on the enhancement of the heat transfer rate. The mathematical model of this system is composed of nonlinear partial equations that the analytical solution for them is very complex, hence the need for numerical analysis is mandatory with the aid of a finite volume method. Accordingly, The numerical results are presented in axial and transverse velocity, temperature, local and average Nusselt number, local friction coefficient, pressure drop, heat transfer rate, and turbulence kinetic energy. The results revealed that it is possible to improve the thermal performance of the considered system by adopting designs that allow the maximum heat transfer rate with the minimum energy loss. In addition, results show that at the lowest Reynolds number (Re = 10,000), as the height of baffles rises from 0.01 to 0.03 m (growth by 200%), the heat transfer rate augments about 59.09%. Moreover, at the highest evaluated Reynolds number (Re = 87,300), by increasing the height of baffles up to 200%, the heat transfer rate increases by approximately 50.53%. Furthermore, employing a higher number of baffles leads to more heat transfer rates and a significant pressure drop.

Numerical implementation and error analysis of nonlinear coupled fractional viscoelastic fluid model with variable heat flux

The studies conducted in the recent past reveals that the non-Newtonian are extensively applied in several engineering applications. The atypical Oldroyd-B fluid, a specific sub-category of non-Newtonian fluids, with fractional derivative constitutive equations, is very expedient to determine the approximation of many dilute polymer liquids. Since very few studies regarding the numerical solution of the aforesaid model are conducted, so the literature that can be referred to in this regard is scarce. The current article investigates the unsteady magneto-hydrodynamic flow of irregular Oldroyd-B fluid with a modified thermal flux restricted between two infinite parallel plates. Here, it is assumed that the thermal conductivity of the fluid is variable, and the impact of body forces like gravity is accordingly analyzed in terms of non-linear convection. The linear acceleration of the lower permeable plate in its plane prompts the flow. Further, we have investigated the heat transfer in the flow field through a modified fractional thermal gradient and internal heat source. The temperature gradient accelerates with time and exists among the flow boundaries. In order to determine the approximate solution unified with finite difference approximation for Caputo fractional time derivatives, we have utilized the standard Galerkin finite element method. Moreover, we have conducted a convergence analysis of the suggested numerical scheme and offered some valid error approximations. In the non-integer viscoelastic model, the local number and coefficient of skin friction are also deliberated. We have also performed some upright numerical simulations to highlight the ascendency of characteristic flow parameters of velocity and temperature fields in the proposed scheme. Here, we have analyzed that the resistive force increases by 40%, corresponding to an increment in α ranging from 0 to 0.4. Further, the heat transfer rate is diminish by 10.94% against an increase in variable thermal conductivity while it is enhanced by 37.78% corresponding to increased in Pr.

Frictional drag reduction caused by bubble injection in a turbulent boundary layer beneath a 36-m-long flat-bottom model ship

The reduction of frictional drag caused by bubble injection was experimentally investigated in a turbulent boundary layer beneath a 36-m-long flat-bottom model ship. For this model ship towed at 8 m/s, the drag reduction increased exponentially (with a power index of 1.2) with the air flow rate, and the drag-reduction ratio reached 28% at the maximum air flow rate. The local wall shear stress measured at 23 locations on the flat-bottom surface revealed a reduction in the local friction coefficient of 50% immediately downstream of the bubble injector and 20% at the stern. This downstream decay proceeded nonlinearly, inferring that the spatial transition of local drag-reduction characteristics depends on the status of bubble dispersion. We established a formula characterizing the streamwise decay from measurements of the transition of drag-reduction characteristics along the 36 m length of the model. This formula can be used to evaluate the downstream persistence of bubbly drag reduction and estimate the full-scale performance of air lubrication technology.

Influence of Metal Working Fluids in Cutting Processes

For the realization of efficient production processes, an understanding of the appropriate application of metal working fluids (MWF) is necessary. In addition to knowledge about the process-related aspect of chip transport and the macroscopic cooling effect, the characteristics and properties of the lubrication film thickness and the cooling conditions in the area of the secondary shear zone on the chip surface, i.e. in the direct vicinity of the material separation, represent a fundamental scientific issue within production technology. In particular, these areas generate a high proportion of heat during machining, so that the local friction phenomena have a significant influence on the resulting edge zone of the produced component and the thermomechanical load on the tool. Currently, there are no numerical models and methods for mapping and predicting the lubrication film thickness that can be used in the sense of a targeted design of the cooling lubricant supply. The aim is to transfer the methods from the field of tribology of machine elements, which have already led to significant knowledge gains in this discipline, to machining and couple them to approaches already established in machining. To this end, experiments on tribometers have been performed as a first step. For example, an oscillating pin-on-plate tribometer was used. In this setup, a steel plate is doing oscillating motion against a fixed ball (diameter of 6 mm) under a defined load. The frictional force is recorded during the test. A MWF in a heated tank is used for the lubricant. Additional investigations on the film thickness were performed on an optical EHL (elasto-hydrodynamic lubrication) tribometer. In this setup, a ball rolls on a glass-disc and the resulting film thickness is measured by interferometry.For comparison, the influence of the MWF on the chip formation process in metal cutting was investigated on a special test rig (machine tool). This test rig allows high speed imaging and force measurements of an orthogonal cutting process while using MWFs. The first results show a reduced contact length between chip and tool as well as lower process forces for processes with MWFs compared to dry cutting processes. In future investigations, this test rig will be applied for the identification of the local friction coefficient between chip and tool. The data gained from the cutting test are compared with the output of the tribological test rigs.

An approach on turbulent flow of pseudo-plastic nanofluids and heat transfer subject to wall slip

The purpose of this paper is to study the turbulent flow of pseudo-plastic nanofluids (PPNF) and heat transfer subject to wall slip. Four types of nanoparticles (Fe3O4, CuO, Cu, Ag) are taken into account in carboxymethyl cellulose (CMC)-water-based fluid. The Prandtl mixing length theory is introduced to divide the turbulent boundary layer into two regions: laminar sub-layer and turbulent region. The numerical solutions are obtained by bvp4c technique. Results show that the wall-slip parameter has important influence on velocity and temperature fields, the variations are more intensive in laminar sub-layer. The local friction coefficient decreases for larger wall-slip parameter and smaller fraction of nanoparticles. And the heat transfer is significantly strengthened (in Nusselt number) for smaller wall-slip parameter and larger fraction of nanoparticles. Moreover, for four different nanofluids, the Ag/CMC nanofluid is more conducive to enhance the turbulent heat transfer than other nanofluids.

進行波制御下の乱流境界層流れに対するLDV計測と抵抗低減メカニズムの解析

We conducted a detailed analysis on the experimental data of the traveling wave control for zero pressure gradient turbulent boundary layer flow in which the drag reduction rate resulted in 7%. The free stream velocity is 2.0 m/s and Reynolds number based on the momentum thickness of the measurement section is 640 - 900 and the frictional Reynolds number is 300 - 340. It was confirmed that the traveling wave parameters; amplitude, wavelength, and wave speed are the same as that of the previous experiment conducted in the channel flow. We calculate laminar, turbulent Reynold shear stress (RSS), and periodic component in the same period with the control of FIK identity to analyze the contribution of local friction coefficient dividedly. As a result, the contribution of the turbulent component to the friction coefficient was decreased by the control while the periodic component was increased. Although the mean convection term was not directory calculated, the fact that the displacement thickness was thickened by the control suggests that this term was modified by the control to reduce skin friction. However, contributions of the RSS for the friction coefficient might be overestimated because some regions with the strong negative RSS which had been found in the previous DNS could not be measured.

Coupled flow and heat transfer of power-law nanofluids on non-isothermal rough rotary disk subjected to magnetic field

We study the coupled flow and heat transfer of power-law nanofluids on a non-isothermal rough rotating disk subjected to a magnetic field. The problem is formulated in terms of specified curvilinear orthogonal coordinate system. An improved BVP4C algorithm is proposed, and numerical solutions are obtained. The influence of volume fraction, types and shapes of nanoparticles, magnetic field and power-law index on the flow, and heat transfer behavior are discussed. The obtained results show that the power-law exponents (PLE), nanoparticle volume fraction (NVF), and magnetic field inclination angle (MFIA) have almost no effects on velocities in the wave surface direction, but have small or significant effects on the azimuth direction. The NVF has remarkable influences on local Nusselt number (LNN) and friction coefficients (FC) in the radial direction and the azimuth direction (AD). The LNN increases with NVF increasing while FC in AD decreases. The types of nanoparticles, magnetic field strength, and inclination have small effects on LNN, but they have remarkable influences on the friction coefficients with positively correlated heat transfer rate, while the inclination is negatively correlated with heat transfer rate. The size of the nanoparticle shape factor is positively correlated with LNN.